1. Field of the Invention
The present invention relates to fuel dilution methods and apparatus for reducing the production of nitrogen oxides during the combustion of fuel gas and combustion air.
2. Description of the Prior Art
Nitrogen oxides (NOx) are produced during the combustion of fuel-air mixtures at high temperatures. An initial, relatively rapid reaction between nitrogen and oxygen occurs predominantly in the combustion zone to produce nitric oxide in accordance with the reaction N2+O2xe2x86x922NO. The nitric oxide (also referred to as xe2x80x9cprompt NOxxe2x80x9d) is further oxidized outside the combustion zone to produce nitrous oxide in accordance with the reaction 2NO+O2xe2x86x922NO2.
Nitrogen oxide emissions are associated with a number of environmental problems including smog formation, acid rain and the like. As a result of the adoption of stringent environmental emission standards by government authorities and agencies, methods and apparatus to suppress the formation of nitrogen oxides in flue gases produced by the combustion of fuel-air mixtures have been developed and used heretofore. For example, methods and apparatus wherein fuel is burned in less than a stoichiometric concentration of oxygen to intentionally produce a reducing environment of CO and H2 have been proposed. This concept has been utilized in staged air burner apparatus wherein the fuel is burned in a deficiency of air in a first zone producing a reducing environment that suppresses NOx formation, and then the remaining portion of air is introduced into a second zone.
Other methods and apparatus have been developed wherein flue gases are combined with fuel or fuel-air mixtures in burner structures to thereby dilute the mixtures and lower their combustion temperatures and the formation of NOx. In another approach, flue gases have been recirculated and mixed with the combustion air supplied to the burner upstream of the burner.
While the above described techniques for reducing NOx emissions with flue gas have been effective in reducing NOx formation and flue gas NOx content, there are certain disadvantages and drawbacks associated with them. For example, in converting existing furnaces (including boilers) to flue gas recirculation, the modification or replacement of the existing burner or burners and/or combustion air blowers and related apparatus is often required. The modifications often result in increased flame spread and other combustion zone changes which require internal alterations to the furnaces in which modified burners are installed. The changes and modifications required often involve substantial capital expenditures, and the modified furnaces and burners are often more difficult and costly to operate and maintain than those they replaced.
Thus, there are continuing needs for improved methods and apparatus for reducing NOx formation and emissions in and from existing furnaces without the substantial modifications and expenditures which have heretofore been required.
The present invention provides methods and apparatus which meet the needs described above and overcome the deficiencies of the prior art. The methods of the present invention for reducing the content of nitrogen oxides in the flue gases produced by the combustion of an at least substantially stoichiometric mixture of fuel gas and combustion air introduced into a burner connected to a furnace are basically comprised of the following steps. The combustion air is conducted to the burner, and a chamber is provided outside of the burner and furnace for mixing flue gases from the furnace with the fuel gas. The fuel gas is discharged in the form of a fuel jet into the mixing chamber so that flue gases from the furnace are drawn into the chamber and mixed with and dilute the fuel gas therein. The flue gases-fuel gas mixture formed in the mixing chamber is conducted to the burner wherein the mixture is combined with the combustion air and burned in the furnace.
The apparatus of this invention can be integrated into an existing burner-furnace system without substantially modifying or replacing existing burners, air blowers and the like and reduces the content of nitrogen oxides in the flue gases produced by the combustion of fuel gas and combustion air in the furnace. At most, the burners may require minor modifications to accommodate the increased mass and reduced pressure of the flue gases-fuel gas mixture, e.g., the replacement of the burner tips.
The apparatus is basically comprised of a mixing chamber which is separate from the burner and furnace for mixing flue gases from the furnace with the fuel gas prior to when the fuel gas is conducted to the burner. The mixing chamber includes a fuel gas inlet for connection to a fuel gas conduit and for forming a fuel jet within the chamber, a flue gases inlet positioned so that flue gases are drawn into the chamber by the fuel jet and a flue gases-fuel gas mixture outlet. A flue gases conduit for connection to the furnace is connected to the flue gases inlet of the chamber, and a flue gases-fuel gas mixture conduit for connection to the burner is connected to the flue gases-fuel gas mixture outlet of the chamber.
It is, therefore, a general object of the present invention to provide fuel dilution methods and apparatus for NOx reduction.